1. Technical Field
The technical field relates to the testing of asphalt mixtures for the purposes, for example, of highway construction and, more particularly, to a method and apparatus for fatigue and viscoelastic property testing using a loaded wheel tester.
2. Description of the Related Arts
Loaded Wheel Testers (LWTs), such as the Asphalt Pavement Analyzer (APA) available from Pavement Technology, Inc. of Covington, Ga. (Pavement Technology), the Hamburg wheel tracking device, and the French LWT, are widely used in the United. States and many parts of the world to evaluate the rut-resistance and moisture susceptibility of asphalt mixtures. Referring to FIG. 1, there is shown the APA (Pavement Technology) testing process for fatigue properties of asphalt mixtures. The process involves using one or multiple loaded wheels to apply a moving load to specimens to simulate traffic loads applied on asphalt pavements. Based on the test results, the fatigue performance of asphalt mixtures can be evaluated.
An Asphalt Pavement Analyzer available from Pavement Technology has evolved over the years. At an early developmental stage of the Asphalt Pavement Analyzer (APA) and referring to FIG. 2(a), which is now one of the commonly used LWTs in the United States, the space under the beam specimen was small (indicated in black) and sometimes insufficient to accommodate the beam deformation caused by the moving wheel load during fatigue testing. At that time and referring to FIG. 2(b), conductive wires were attached to the bottom surface of the beam specimen with molten asphalt to detect the fatigue cracking of an asphalt beam test specimen. The fatigue is detected by the conductive wire breaking and demonstrating an open electric circuit. As the deformation at the bottom of the beam increased, the bottom surface of the sagging beam would come into contact with the rigid bottom plate and further vertical deformation would be stopped by the bottom plate; see FIG. 2(a). Under this situation, the conductive wires would not break, the final failure of the beam would be difficult to achieve or, if the conductive wire did break, the break in the wire might not be caused by fatigue cracking of the asphalt mixtures under test.
To enhance the simulation of fatigue cracking, the current version of the APA available from Pavement Technology has a deeper space under beam specimens, which can accommodate more deformation for beam specimens which depth is selected according to the length of the beam specimen under test; (see FIG. 1).
Moreover, viscoelastic testing was not provided for with an APA LWT. The viscoelastic properties of asphalt mixture have been the subject of many studies for several decades. A number of methods and analysis models, have been substantially developed to characterize the viscoelastic response of asphalt mixtures as well. Viscoelasticity as used herein may be defined, for example, as the property of an asphalt mixture to exhibit both viscous and elastic characteristics when undergoing vehicular traffic and environmental phenomenon but this definition is not intended to be limited herein. Due to the inherent nature of viscoelastic materials, the fundamental property that governs the responses caused by external loading is a function of time or loading frequency. Linear viscoelastic behavior for asphalt mixtures may be determined through experimental testing within the linear viscoelastic region, such as creep, relaxation, and complex modulus tests. Due to the challenge of controlling a relaxation test, a creep test is more accepted by researchers based on the interchangeability of the results from both tests. The creep test involves measuring the time dependent strain (e.g. deformation) induced from the application of a constant uniaxial stress, σ0. Creep compliance is defined as the ratio of the time-dependent strain to the constant stress. The creep compliance is a crucial factor for determining the suitability of asphalt concrete under various loading and environmental conditions. Moreover, once the creep compliance is determined, the stress-stain relationship can be expressed with hereditary integral
                              ɛ          ⁡                      (            t            )                          =                                            σ              i                        ⁢                          J              ⁡                              (                t                )                                              +                                    ∫              0              t                        ⁢                                          J                ⁡                                  (                                      t                    -                                          t                      ′                                                        )                                            ⁢                                                ∂                                      σ                    ′                                                                    ∂                                      t                    ′                                                              ⁢                                                          ⁢                              ⅆ                                  t                  ′                                                                                        (        1.1        )            where, ε(t)=strain; σt=initial stress; t′=integration variable related to time.
The complex modulus test is a fundamental test that characterizes the viscoelastic properties of asphalt mixtures. It is considered as a mechanistically based laboratory test to characterize the stiffness and loading resistance of asphalt mixtures. Complex modulus, E*, is composed of real and imaginary parts that define the elastic and viscous behavior for viscoelastic materials. Dynamic modulus, |E*|, obtained from the test is a fundamental property for describing the stress-strain relationship of asphalt mixtures, while phase angle, δ, is a major factor reflecting the viscous behavior of asphalt mixtures which indicates whether the asphalt material is predominantly elastic or viscous.
                              E          *                =                                            σ              amp                        ⁢                          ⅇ                              ⅈω                ⁢                                                                  ⁢                t                                                                        ɛ              amp                        ⁢                          ⅇ                              ⅈ                ⁡                                  (                                                            ω                      ⁢                                                                                          ⁢                      t                                        -                    δ                                    )                                                                                        (        1.2        )                                                                E            *                                    =                              σ            amp                                ɛ            amp                                              (        1.3        )                                          δ          =                      2            ⁢            π                          ⁣                              ·            f            ·            Δ                    ⁢                                          ⁢          t                                    (        1.4        )            where, σamp=amplitude of sinusoidal stress; εamp=amplitude of sinusoidal strain; ω=angular velocity; i=imaginary component; f=loading frequency; Δt=time lag between stress and strain.
Dynamic modulus values measured over a range of temperatures and frequencies of loading can be shifted into a master curve based on a time-temperature superposition principle. The master curve of an asphalt mixture allows comparisons to be made over extended ranges of frequencies and temperature, so that dynamic modulus can be used as an important viscoelastic parameter for performance analysis of asphalt mixtures using constitutive models. Besides, dynamic modulus is also a crucial parameter for pavement design. Most of the researches indicate that any process that results in the use of asphalt mixtures with better selection of dynamic modulus will improve the performance of the pavement.
Although many factors have been proved to have significant effects on the viscoelastic behavior of asphalt material, such as loading magnitude, rate of loading (loading frequency), and temperature variations, there are only a few direct evidences or relative works regarding evaluation of the effect of the loading mode (e.g. tension, tension/compression and compression). Through testing loading conditions different from the actual states, significant errors and unreasonable design may occur. Currently, several testing methods and devices have been created to investigate the viscoelastic properties of asphalt concrete based on creep and complex modulus tests in all kinds of testing situations. According to the fundamental stress and strain situation in the asphalt pavement, the fatigue life of a particular asphalt pavement mixture is primarily determined by the tensile properties of the asphalt mixture it comprises. Therefore, it is more appropriate to use the parameters obtained from a tension test to evaluate the performance of asphalt concrete. In fact, a pavement structure is subjected to a triaxial stress state under actual vehicular traffic loading. As a continuous medium, pavement structure tends to spread the stress out received from the vehicular traffic in all directions. With the development of better testing equipment and analysis methods, it becomes possible to better simulate the stress state of pavement structure in laboratory testing. Although all kinds of stress states such as uniaxial, biaxial, and triaxial can be simulated in the laboratory, the real stress state that exists in the pavement cannot be achieved. The development of testing method is still the bottleneck for achieving a better and clearer understanding of pavement material properties.
For testing viscoelastic properties, three known tests are shown in FIGS. 5(a), (b) and (c). These include the Direct Tension Test (DTT) for testing asphalt concrete in tension depicted in FIG. 5(a). A unique benefit of the DTT test is that the stress of the specimen is in a uniaxial tension state which makes the stress-strain analysis much simpler. The Simple Performance Test depicted in FIG. 5(b), also known as the asphalt mixture performance test (AMPT), tests a cylindrical specimen for controlled sinusoidal stress loading at various loading frequencies and test temperatures. An indirect tension (IDT) testing method, depicted in FIG. 5(c), characterizes Poisson's ratio, creep compliance, resilient modulus and splitting tensile strength of asphalt mixtures by subjecting the depicted cylindrical specimen to a diametrical load.
Consequently, there is an opportunity to improve test apparatus and a method for asphalt mixtures using a loaded wheel tester or related apparatus.